During the fabrication of some integrated circuits, a moat opening may be formed, within which is grown a field isolation oxide to electrically separate active areas of the semiconductor wafer. A transistor may be formed in each active area, and each transistor is electrically isolated from the other by the field isolation oxide layer. This method of oxidation is known as local oxidation of silicon (LOCOS).
A field isolation oxide layer can be grown in a semiconductor wafer by first forming a layer of silicon nitride (Si.sub.3 N.sub.4) over the semiconductor wafer. A photolithographic mask is applied to the silicon nitride layer, and the exposed silicon nitride layer is etched away in the areas corresponding to the openings in the photolithographic mask. The photolithographic mask may comprise a photoresist layer. The exposed portions of the silicon nitride layer may be etched away using reactive ion etch techniques. After the exposed portions of the silicon nitride layer are removed, the photoresist layer is removed, and a field isolation oxide layer is grown in the etched openings of the silicon nitride layer. After the growth of the field isolation oxide layer, the silicon nitride layer is removed, leaving a field isolation oxide layer serving as an isolation structure between two active regions of the semiconductor wafer.
The etch bias of an etched silicon nitride opening is the difference between the width of the etched silicon nitride opening and the corresponding opening in the photoresist mask. An optimal etch bias is zero, indicating that the silicon nitride opening and the initial photoresist mask opening are the same width. A positive etch bias indicates that the etched silicon nitride opening is wider than the corresponding opening in the photoresist mask.
Problems encountered when etching the moat opening include the difficulty of achieving an etch bias of zero. In order to accurately create field isolation oxide layers, the process of etching the silicon nitride must accurately reproduce the dimensions of the photoresist mask in the silicon nitride layer. The dimensions of the etched moat opening in the silicon nitride layer must ideally be the same as the initial dimensions of the corresponding opening in the overlaying photoresist mask. Thus, for design purposes, it is important that the moat openings in the silicon nitride layer have predictable dimensions.
The etching of the silicon nitride layer is often accomplished in an ion-assisted plasma reactor. During the etching process, the chemical content of the plasma may erode the sidewalls of the photoresist mask opening. The gradual and continued erosion of the sidewalls of the photoresist layer widens the openings in the photoresist mask, thus providing a wider angle for ion bombardment in the reactor. Increased ion bombardment on the sidewalls of the photoresist layer and the silicon nitride layer leads to increased erosion of the sidewalls of the photoresist layer and the silicon nitride layer and results in a silicon nitride etch with an undesirably high etch bias.
Inaccurate etching of the silicon nitride layer is often exacerbated by the seasoning of the plasma reactor. The chemical content, or seasoning, of the walls of the plasma reactor changes after each etch, thereby hindering the accuracy of the silicon nitride etch.